1. Field of the Invention
The present invention concerns a electron power tube, for example of the triode, tetrode or pentode type, cooled by the circulation of a fluid. In particular, it concerns means to improve the cooling of the anode connection part.
2. Description of the Prior Art
In electron tubes of this type, a cathode emits a flow of electrons towards an anode. This flow of electrons is modulated by one or more grids before reaching the anode. The kinetic energy of the electrons is converted into heat at the anode. For electron power tubes, the energy to be dissipated may be so high that it becomes necessary to use cooling devices working with a fluid under forced circulation. This fluid is often air, for low power values, and a liquid, notably water, for high power values.
Power electron tubes generally have the following configuration. In the case of a triode with directly heated cathode, for example: the cathode is shaped like a cylinder having, for its axis, the longitudinal axis of the tube. Then, there is a grid that surrounds the cathode and, finally, an anode which surrounds the grid. The heated part of the cathode constitutes, along the longitudinal axis, an active electron-emitting length: the electrons go through the grid along radial directions and are picked up by the anode. The anode and the grid are each shaped like a hollow cylinder, and each of them has, at its base, on the foot side of the tube, a cylindrical part for external electrical connection. These anode and grid connection parts are solidly joined together, mechanically, by being sealed to one and the same insulating tie. The anode forms a part of the imperviously sealed chamber, within which is set up the vacuum needed for the working of the electron tube.
The tube is cooled by means of a cooler, formed by two coaxial, cylindrical jackets mounted around the anode, one end of the outer jacket being fixed imperviously to a flange of the tube, located on the foot side of this tube. The outer jacket is provided with an inlet conduit through which comes the cooling fluid, water for example.
The circulation of water is forced, and the water circulates, for example, between the outer jacket and the inner jacket up to the base of the external jacket, where the water is injected and advances between the space formed between the inner jacket and the external wall of the anode. The transfer of calories to the water takes place in this zone. The water is then removed by means of a second conduit with which the inner jacket is provided.
The efficiency with which the anode is cooled is related, in a manner known per se , to the characteristics of flow of the cooling fluid, notably to the pressure with which the water is injected into the cooler and to the dimensions of the space btween the inner jacket and the external wall of the anode.
The above-described configuration has one disadvantage: the elements located near the anode tend to undergo a high rise in temperature due to thermal conduction. This is the case, in particular, for the anode connection part.
The anode connection part provides both the external electrical connection of the anode and the mechanical connection with the insulating tie that separates it from the grid connection part. This insulating tie is prefer ably made of ceramic.
The microwave currents circulate on the surface of the conductors, on a thickness which is all the smaller as their frequency is high. This phenomenon is known as skin effect. The rise in the frequency leads to an increase in losses by Joule effect, more particularly at the connection between the anode connection part and the insulating tie.
For, at this place, because of the metal/ceramic sealing, the level of losses can be reduced only by resorting, as in the rest of the part, to a surface coating which is a good conductor of electricity, such as copper for example.
The intense heating of the anode connection part may be detrimental to the electron tube. This heating is due to heat conduction and to Joule effect. The materials forming the anode connection part and the insulating tie, namely the metal and the ceramic, have different behaviour characteristics the more the temperature rises. The mechanical strains induced in these materials then entail the risk of causing breakage either of the metal/ceramic sealing or in the insulating space itself.